In addition to methane, natural gas includes some heavier hydrocarbons with impurities, e.g., carbon dioxide, nitrogen, helium, water, and non-hydrocarbon acid gases. After compression and separation of these impurities, natural gas is further processed to separate and recover natural gas liquids (NGL). In fact, natural gas may include up to about fifty percent by volume of heavier hydrocarbons recovered as NGL. These heavier hydrocarbons must be separated from methane to be recovered as natural gas liquids. These valuable natural gas liquids consist of ethane, propane, butane, and other heavier hydrocarbons. In addition to these NGL components, other components including hydrogen, ethylene, and propylene may be contained in gas streams obtained from refineries or from petrochemical plants.
Processes for separating hydrocarbon gas components are well known in the art. C. Collins, R. J. Chen, and D. G. Elliot have provided an excellent general review of NGL recovery methods in a paper presented at Gas Tech LNG/LPG Conference 84. This paper, entitled “Trends in NGL recovery for natural and associated gases”, was published by Gas Tech, Ltd. of Rickmansworth, England, in the transactions of the conference on pages 287-303. In addition, R. J. Lee, J. Yao, and D. G. Elliot provided an excellent general review of NGL recovery methods in a paper entitled “Flexibility, efficiency to characterize gas-processing technologies”, which was published in the Dec. 13, 1999 issue of Oil & Gas Journal on pages 90-94. The pre-purified natural gas is treated by well-known methods including absorption, refrigerated absorption, adsorption and condensation at cryogenic temperatures down to −175° F. Separation of the lower hydrocarbons is achieved in one or more distillation towers. The columns are often referred to as demethanizer or deethanizer columns. Processes employing a demethanizer column separate methane and other volatile components from ethane and heavier (C2+) components in the purified natural gas liquids. The methane fraction is recovered as purified gas for pipeline delivery. Ethane and less volatile components, including propane, are recovered as natural gas liquids. In some applications, however, it is desirable to minimize the ethane content of the NGL. In those applications, ethane and more volatile components are separated from propane and less volatile (C3+) components in a column generally called the deethanizer column.
NGL recovery plant design is highly dependent on the operating pressure of the distillation column. At medium to low pressures, i.e., 400 psia or lower, the recompression horsepower requirement (to compress the residue gas to pipeline pressure) will be so high that the process becomes less economical. However, at higher pressures, the recovery level of the hydrocarbons will be significantly reduced due to the less favorable separating conditions, i.e., lower relative volatility inside the distillation column. Prior art has concentrated on operating the distillation columns at a higher pressure, i.e., 400 psia or higher while maintaining the high recovery of liquid hydrocarbons.
Many patents have been directed to methods for improving this separation technology. U.S. Pat. Nos. 4,171,964, 4,278,457, 4,687,499, and 4,851,020 describe relevant processes.
While single-column processes utilizing only the demethanizer have been capable of recovering more than 98% of the propane, propylene, and heavier hydrocarbons during the ethane recovery mode, most of those processes fail to maintain the same propane recovery level when ethane is not needed and operated in the ethane rejection mode. Due to equilibrium constraints, the propane recovery in a single-column arrangement is ultimately limited by propane content in the top reflux to the demethanizer. To overcome this deficiency, various methods employing sequentially configured first and second distillation columns, e.g. a demethanizer followed by a deethanizer, are disclosed. In this arrangement, the overhead vapor from the second column is condensed and recycled to the top of the first column as the reflux. The top reflux thus derived is essentially propane-free, thereby enhancing propane recovery efficiency.
In the afore-mentioned two-column arrangement, most prior art uses the first column comprising only the rectification section like an absorber. The absorber bottom liquids are transported to the second column for further processing to generate a reflux lean in propane for use as the top reflux to the first column. For examples, see U.S. Pat. Nos. 4,617,039, 4,690,702, 5,771,712, 5,890,378, 6,601,406, 6,712,880, and 6,837,070. An improved two-column scheme disclosed in U.S. Pat. No. 6,116,050 thermally links both distillation columns via a side reboiler-overhead condenser and introduces a stripping section to the first column. The provision of the stripping section allows undesirable light components to be stripped off the liquids feeding to the second column.
A significant cost in the NGL recovery processes is related to the refrigeration required to chill the inlet gas. Refrigeration for these low temperature schemes is generally provided by using propane or ethane as refrigerants. In some applications, mixed refrigerants and cascade refrigeration cycle have been used. Refrigeration is also provided by turbo expansion or work expansion of the compressed natural gas feed with appropriate heat exchange.
Traditionally, the gas stream is partially condensed at medium to high pressures with the help of external propane refrigeration, a turboexpander or both. The condensed streams are further processed in a distillation column, e.g., a demethanizer or deethanizer, operated at medium to low pressures to separate the lighter components from the recovered hydrocarbon liquids. Turboexpander technology has been widely used in the last 30 years to achieve high ethane and propane recoveries in the NGL for leaner gases. For rich gases containing significant quantities of heavy hydrocarbons, a combined process of turboexpander and external refrigeration is the most efficient approach. Mechanical refrigeration consisting primarily of a pure refrigerant and in closed circuit, such as propane, is commonly used as the source of external refrigeration in cryogenic turbo expansion processes.
In addition to the external propane refrigeration, the use of an auto-refrigeration system has been utilized in prior art. U.S. Pat. No. 5,588,308 discloses that NGL product is recovered by cooling and partial condensation of a purified natural gas feed wherein a portion of the necessary feed cooling and condensation duty is provided by expansion and vaporization of condensed feed liquid after methane stripping, thereby yielding a vaporized NGL product. Additional refrigeration for feed gas cooling is provided by vaporizing methane-stripped liquid, which in turn provides boilup vapor for the stripping step. This process eliminates the need for external refrigeration with the aid of auto-refrigeration and integrated heat exchange.
U.S. Pat. No. 5,992,175 introduces a self-refrigeration scheme in open cycle to improve the efficiency and economy of processes for the recovery of natural gas liquids (NGL) from a gas feed under pressure. In this process, a portion of a hydrocarbon liquid is withdrawn from the lower portion of a distillation column. This withdrawn liquid hydrocarbon is expanded and heated to produce a two-phase system for separation into a heavy, liquid hydrocarbon product and a vapor phase for recycling to the column, preferably as an enhancement vapor. The withdrawn hydrocarbon liquid is preferably heated by indirect heat exchange with the inlet gas, thus reducing or eliminating the external refrigeration requirements of the process. The expanded, heated vapor recycled to the column increases the ethane and propane concentration in the column, thus reducing the tray temperature profile and increasing the separation efficiency. Accordingly, the column may be operated at higher pressures while maintaining the same separation efficiency, resulting in significant energy savings and economies of operation.
The open refrigeration cycle disclosed in the '175 patent not only reduces the requirements for external refrigeration, but also provides essentially all the reboiler duty for the distillation column. However, the reboiler duty required for the distillation column is generally limited to the specific distillation objective. Therefore, the refrigeration that can be effectively employed by this technique is somewhat restricted. In some cases where a large amount of external refrigeration is needed, there seems to be a shortage of refrigeration that can be produced via the above technique as a result of this limitation. This is particularly true for a relatively rich gas.
As can be seen from the foregoing description, prior art has long sought methods for improving efficiency and economics of processes for separating and recovering natural gas liquids from natural gas. Accordingly, there has been a long-felt but unfulfilled need for more efficient and more economical methods for performing this separation. The present invention provides significant improvements in efficiency and economy, thus solving those needs.